1. Field of the Invention
This invention pertains to composite parts prepared from oriented yarns of high modulus embedded in a resin matrix, which is subsequently cured, to give a lightweight composite part of high strength.
2. Background of the Prior Art
The aerospace industry is making increasing use of composite materials, used herein to denote materials prepared from oriented yarns of high modulus embedded in a cured resin matrix, as high strength, lightweight parts useful wherever energy conservation, and high performance standards are required. As a result, more and more parts, and varied types of parts, prepared from these materials are being integrated in the overall structures of aircraft, spacecraft and the like. Naturally, increasing numbers of these structures have complex, or non-planar, shapes.
A number of methods are known for the preparation of these composite parts. One method involves the use of "prepregs", or unidirectional tapes impregnated with a curable resin, which are then "laid-up", or formed around or in a mold, and subsequently trimmed and cured, to the desired shape and characteristics.
An entirely different method, embodied in U.S. Pat. Nos. 4,664,961 and 4,681,049, involves the preparation of dry "fiber forms", or a three dimensional array of layers or bundles of oriented, high modulus yarns, the orientation being prepared according to a predetermined schedule, which approximate the shape of the final product desired. This fiber form is infused or saturated with a curable resin, and then subjected to curing conditions, and trimmed to final dimensions, to provide the desired composite article., A similar approach is disclosed in U.S. Pat. No. 4,410,577, using woven layers having different yarn orientation.
When prepared according to the latter process described above, that is, through the use of a dry fiber form, the fiber form is frequently prepared by marrying sub-assemblies, to approximate the final shape. Thus, a "T" shape, as illustrated in FIG. 1, can be visualized as the marriage of three sub-assemblies, pictured in FIG. 2, including two "L" shapes X and Y, and straight flange Z. This is applicable not only to the T-shaped part illustrated herein, but virtually any composite fiber form characterized by at least two portions which lie in distinct, non-parallel planes, the two portions being integral and joined at a common juncture.
In the above-described method, and indeed, other methods known for the assembly of dry fiberforms, this inevitably results in a central area A at the plane juncture is interface of two distinct planes, applied load in any of several directions, "in plane" or out, can result in stresses which are "out-of-plane", or at an angle to the fiber direction. One such direction may be at a direction normal to the tangent described by the fibers at the junction interface. Due to lack of reinforcement running in this direction at the interface, cracking may occur. The resulting failure mode is a complicated dynamic involving both shear and cracking.
Accordingly, the provision of composite structural parts of complex shape, prepared using dry fiber forms which are subsequently infused with a resin and cured, which do not have a central or juncture section, characterized by a lack of reinforcement in "out-of-plane" directions of stress remains an unsatisfied need of the prior art.